This invention relates generally to magnetic recording media, and more particularly to thermally stable high density media.
Conventional magnetic recording media, such as the magnetic recording disks in hard disk drives, typically use a granular ferromagnetic layer, such as a sputter-deposited cobalt-platinum (CoPt) alloy, as the recording medium. Each magnetized domain in the magnetic layer is comprised of many small magnetic grains. The transitions between magnetized domains represent the xe2x80x9cbitsxe2x80x9d of the recorded data. IBM""s U.S. Pat. Nos. 4,789,598 and 5,523,173 describe this type of conventional rigid disk.
As the storage density of magnetic recording disks has increased, the product of the remanent magnetization Mr (the magnetic moment per unit volume of ferromagnetic material) and the magnetic layer thickness t has decreased. Similarly, the coercive field or coercivity (Hc) of the magnetic layer has increased. This has led to a decrease in the ratio Mrt/Hc. To achieve the reduction in Mrt, the thickness t of the magnetic layer can be reduced, but only to a limit because the layer will exhibit increasing magnetic decay, which has been attributed to thermal activation of small magnetic grains (the superparamagnetic effect). The thermal stability of a magnetic grain is to a large extent determined by KuV, where Ku is the magnetic anisotropy constant of the layer and V is the volume of the magnetic grain. As the layer thickness is decreased, V decreases. If the layer thickness is too thin, the stored magnetic information will no longer be stable at normal disk drive operating conditions.
One approach to the solution of this problem is to move to a higher anisotropy material (higher Ku). However, the increase in Ku is limited by the point where the coercivity Hc, which is approximately equal to Ku/Mr, becomes too great to be written by a conventional recording head. A similar approach is to reduce the Mr of the magnetic layer for a fixed layer thickness, but this is also limited by the coercivity that can be written. Another solution is to increase the intergranular exchange, so that the effective magnetic volume V of the magnetic grains is increased. However, this approach has been shown to be deleterious to the intrinsic signal-to-noise ratio (SNR) of the magnetic layer.
Magnetic recording media with high intrinsic SNR (low intrinsic media noise) is desirable because it is well known in metal alloy media, such as CoPt alloys, that the intrinsic media noise increases with increasing linear recording density. Media noise arises from irregularities in the magnetic transitions and results in random shifts of the readback signal peaks. These random shifts are referred to as xe2x80x9cpeak jitterxe2x80x9d or xe2x80x9ctime jitterxe2x80x9d. Thus higher media noise leads to higher bit error rates. It is therefore desirable to develop a thin film metal alloy magnetic media that generates noise below a maximum acceptable level so that data can be recorded at maximum linear density. It is known that substantially improved SNR can be achieved by replacing a single magnetic layer with a laminated magnetic layer of two (or more) separate magnetic layers that are spaced apart by an nonmagnetic spacer layer. This discovery was made by S. E. Lambert, et al., xe2x80x9cReduction of Media Noise in Thin Film Metal Media by Laminationxe2x80x9d, IEEE Transactions on Magnetics, Vol. 26, No. 5, September 1990, pp. 2706-2709, and subsequently patented in IBM""s U.S. Pat. No. 5,051,288. The reduction in media noise by lamination is believed due to a decoupling of the magnetic interaction or exchange coupling between the magnetic layers in the laminate. The use of lamination for noise reduction has been extensively studied to find the favorable spacer layer materials, including Cr, CrV, Mo and Ru, and spacer layer thicknesses, from 5 to 400 A, that result in the best decoupling of the magnetic layers, and thus the lowest media noise. This work has been reported in papers by E. S. Murdock, et al., xe2x80x9cNoise Properties of Multilayered Co-Alloy Magnetic Recording Mediaxe2x80x9d, IEEE Transactions on Magnetics, Vol. 26, No. 5, September 1990, pp. 2700-2705; A. Murayama, et al., xe2x80x9cInterlayer Exchange Coupling in Co/Cr/Co Double-Layered Recording Films Studied by Spin-Wave Brillouin Scatteringxe2x80x9d, IEEE Transactions on Magnetics, Vol. 27, No. 6, November 1991, pp. 5064-5066; and S. E. Lambert, et al., xe2x80x9cLaminated Media Noise for High Density Recordingxe2x80x9d, IEEE Transactions on Magnetics, Vol. 29, No. 1, January 1993, pp. 223-229. U.S. Pat. No. 5,462,796 and the related paper by E. Teng et al., xe2x80x9cFlash Chromium Interlayer for High Performance Disks with Superior Noise and Coercivity Squarenessxe2x80x9d, IEEE Transactions on Magnetics, Vol. 29, No. 6, November 1993, pp. 3679-3681, describe a laminated low-noise disk that uses a discontinuous Cr film that is thick enough to reduce the exchange coupling between the two magnetic layers in the laminate but is so thin that the two magnetic layers are not physically separated.
What is needed is magnetic recording media that will support very high density recording while retaining good thermal stability and SNR.
The invention is a laminated medium for horizontal magnetic recording that includes an antiferromagnetically (AF)-coupled magnetic layer as one of the individual magnetic layers and a conventional single magnetic layer as the other individual magnetic layer, with the two magnetic layers separated by a nonferromagnetic spacer layer. The AF-coupled magnetic layer of the laminated medium has two ferromagnetic films exchange coupled antiferromagnetically across a nonferromagnetic spacer film and a net remanent magnetization-thickness product (Mrt) which is the difference in the Mrt values of the two ferromagnetic films. In the AF-coupled magnetic layer, the thickness of the nonferromagnetic spacer film is selected to maximize the strength of the antiferromagnetic exchange coupling between the two ferromagnetic films, resulting in antiparallel alignment of the magnetic moments of the two ferromagnetic films. The individual grains in the ferromagnetic films possess dipole fields which also contribute to the coupling across the spacer film and favor this antiparallel moment alignment. However, in the AF-coupled magnetic layer the strength of the dipole coupling fields is substantially less than the exchange field from the antiferromagnetic exchange coupling.
In contrast to the nonferromagnetic spacer film in the AF-coupled magnetic layer, the nonferromagnetic spacer layer used to separate the AF-coupled magnetic layer and the single magnetic layer has a thickness to assure that the single magnetic layer is not exchange coupled antiferromagnetically to the nearest ferromagnetic film of the AF-coupled magnetic layer, even though dipole fields are present and can favor antiparallel moment alignment. However, because the dipole fields across the thicker nonferromagnetic spacer layer are less than the coercive field of either the AF-coupled magnetic layer or the single magnetic layer, the magnetic moment of the single magnetic layer and the net magnetic moment of the AF-coupled magnetic layer are oriented parallel in the remanent magnetic states after being saturated in an applied magnetic field.
The AF-coupled magnetic layer can be located below or above the single magnetic layer, with the two magnetic layers separated by the nonferromagnetic spacer layer. The laminated medium can include one or more AF-coupled layers and one or more single magnetic layers in the laminate.
For a fuller understanding of the nature and advantages of the present invention, reference should be made to the following detailed description taken together with the accompanying figures.